[time-nuts] establishing your position w/o gps

Eric Garner garnere at gmail.com
Tue Jan 24 14:32:57 EST 2012


I managed to score a Davis Mark 25 sextant off of ebay for under 100
dollars. it's taken a fairly large amount of practice but I'm able to
get my position to a bit north of a mile.

It's a fun skill to acquire, and  has lead me off in several other fun
hobby tangents.

-Eric

On Tue, Jan 24, 2012 at 10:13 AM, Bob Camp <lists at rtty.us> wrote:
> Hi
>
> If you spend some time on the auction sites you can find some fairly good
> (though not brand name) sextants on the cheap.
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Chris Albertson
> Sent: Tuesday, January 24, 2012 12:48 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] establishing your position w/o gps
>
> If you want to try your hand at position determination in the pre
> radio nav days you can buy a "studen sextent"  It's a low cost plastic
> instrument sells for about $60.  Better ones start at $200 with $500
> to $800 for a good one.   But it required much pratice and training to
> outgrow the plastic instrument.    I took the class.    I think most
> anyone who wants to sail on the ocean had better take the class just
> in case their GPS fails.   I know some one who had both his primary
> and backup GPSes fail and he was still a week from Hawaii.   They had
> to revert to the old techniques from the 1700's
>
> Much of pre-GPS position determination is not about finding your
> latitude and longitude.  That is a modern notion.    What they did and
> what sailors still do is find a "line of position".  That means "I am
> some place on this line but I don't know where on the line"  There are
> many ways to do this and they would work every method and find several
> lines.   If they could see land they could shoot a compass bearing and
> draw a reciprocal bearing and know they were on that line.  They would
> know the ship's heading and could estimate drift and know course over
> ground was parallel to that.  They could always find a latitude line.
>  Then if they did this right some of these lines would roughly
> intersect and they would know the position without need to know
> longitude.   There were other methods to find lines that required an
> estimate of your speed and without clocks they resorted to chants and
> songs (jo ho, jo ho,...)  As long as you sing the old pirate song at
> the same tempo every time you have a decent clock.  Then you measure
> distance by tossing a big chunk of lumber overboard with a measured
> rope tied to it.  The captains hated doing math by hand so they
> calibrated the rope by tieing knots at intervals so the natural unit
> was one arc minute at the equator and called it a "knot".
>
> My buddy who was headed to hawaii put both GPSes in the oven in the
> galley and after three days was able to get one of them to work a few
> minutes  a couple times a day.  That was enough.  But he said he was
> within maybe 15 miles of where he thought he was
>
> Basically your estimated course line intersected with a line of
> latitude gives you longitude.
>
>
>
>
>
>
> On Tue, Jan 24, 2012 at 8:50 AM, Azelio Boriani
> <azelio.boriani at screen.it> wrote:
>> Yes, the first real push was the Longitude Act (1714) and the Harrison's
>> clocks.
>>
>> On Tue, Jan 24, 2012 at 5:36 PM, Chris Albertson
>> <albertson.chris at gmail.com>wrote:
>>
>>> On Tue, Jan 24, 2012 at 3:16 AM, Attila Kinali <attila at kinali.ch> wrote:
>>> > On Tue, 24 Jan 2012 11:04:08 +0000
>>> > "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
>>> >
>>> >> In message <20120124115848.312d60bd4fccce4f3e71c136 at kinali.ch>, Attila
>>> Kinali w
>>> >> rites:
>>> >>
>>> >> >All this talk about telling the time using stars or the sun made me
>>> wonder
>>> >> >how did people tell what position their telescopes had back in the
> days
>>> >> >before GPS?
>>>
>>> Sailingships and trade was what pushed this.   At the time of Columbus
>>> he was able to know his latitude within a few 10s of miles but even
>>> after returning to Europe he did no know how far around the world he
>>> had sailed.  Was it 1/3rd or 2/3rds?  They had no way to know.    The
>>> problem was that on one had a clock that should keep time well enough.
>>>  They used hour glasses on board ship for short duration time keeping
>>> but those were of no use on a longer ocean crossing.
>>>
>>> Later they discovered the idea of common view of the moons of Jupiter
>>> and they could measure the time from local noon some even on Jupitor
>>> while a person back home did the same thing.  Later when he got back
>>> home they compare notes and then know the difference in longitude.
>>>  Good ocean going clocks were still centuries away.    But in the
>>> 1500's they could only know the location after the fact when they
>>> returned
>>>
>>> --
>>>
>>> Chris Albertson
>>> Redondo Beach, California
>>>
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>
>
> --
>
> Chris Albertson
> Redondo Beach, California
>
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-- 
--Eric
_________________________________________
Eric Garner




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